The present invention relates to the speed control for electric motors and more particularly but not exclusively the invention relates to control of the speed of a low inertia, fast response spindle motor driving a light weight recording disk, particularly of the flexible type commonly referred to also as floppy disk.
Motor speed control, particularly for d.c. motors is a well developed art. Usually the control circuit includes a tachometer feedback providing a signal that represents the speed of the motor. An error signal is formed in some fashion under utilization of a reference signal from an appropriate source, and the motor is controlled in response to the comparison between feedback reference signals. Generally speaking, a choice is sometimes available between a high inertia, slow response motor, wherein the speed is kept constant to a considerable extent by fly wheel effect; and a low inertia, fast response motor in which constancy of speed is primarily determined by active control of the motor. The invention is primarily related to improvements in the latter type of control.
Generally speaking, the feedback control for motors employs in some instances the principle of frequency shift and discrimination. The feedback signal developed in such a system is an oscillation signal whose frequency changes with motor speed. By comparing the frequency with a standard, speed changes can be detected, and the detected deviation is used in some fashion to control the motor. Among the elements used for obtaining the a.c. feedback is an optical tachometer disk having markings which are optically scanned, and the alterations in contrast are translated by the scanner into an a.c. signal. The invention does relate to that kind of feedback.
The motors used for driving a flexible disk are controlled, for example, by way of pulses to obtain the desired degree of accuracy. It was found, however, that a motor when controlled in this fashion runs rather noisely.